Composite structure having a porous filter medium and a method for forming same

ABSTRACT

A composite structure comprises a porous filter medium, a substrate provided with at least one drainage pathway, and a support and drainage medium sandwiched between the porous filter medium and the substrate. The porous filter medium, the support and drainage medium, and the substrate are bonded free of any adhesive.

This application is a 35 U.S.C. §371 filing of International ApplicationPCT/US94/10942, published as WO96/09879 Apr. 4, 1996, which in turn is acontinuation-in-part of U.S. Pat. application Ser. No. 08/038,257, filedMar. 24, 1993, now U.S. Pat. No. 5,458,719, which is incorporated hereinby reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method of forming a compositestructure. More specifically, the invention relates to a method forbonding a porous medium and a support and drainage medium to the surfaceof a substrate.

BACKGROUND OF THE INVENTION

Porous media are bonded to the surface of a substrate for a wide varietyof purposes. For many applications, such as the formation of a resilientor acoustically absorptive surface, neither the nature of the materialwhich accomplishes the bond nor the depth to which the bond penetratesthe porous facing is critical.

For a wide range of other applications, such as the purification ofpharmaceutical fluids or the removal of bacteria from foods, e.g., milkand beer, bonded assemblies which include finely porous filter mediasecured to a solid substrate are used. Secure bonding of the porousmedium to a solid substrate is particularly necessary when the porousmedium is exposed during service to very high shear forces which woulddisrupt an unsupported membrane.

Filtration applications also typically require that the porous medium bebonded to the substrate such that the fluid passing through the membraneis provided with passageways through which it can flow as it issues fromthe membrane. Typically, the passageways are grooves cut or cast into aplane surface, the grooves being configured to drain collectively into acentral outlet port, which the user connects to a receiver for thefiltrate.

The porous medium may be secured to the substrate by applying a layer ofa viscous adhesive to the substrate and then contacting the porousmedium with the adhesive layer. The use of a third component, i.e., theadhesive, which could leach into the filtrate, is very undesirable formany of the applications described above. In addition, the adhesive canoften blind a substantial number of the pores and alter the permeabilityof the medium.

Bonded assemblies may also be produced by contemporaneously forming andintegrally securing a porous medium to the surface of a substrate. Thismethod, however, is severely limited by the requirement that the porousmedium be precipitated from a liquid suspension and secured to thesubstrate in a single step. Some porous media, which may be employedeffectively in filter applications, are not formed from liquidsuspension. For example, polytetrafluoroethylene (e.g. Teflon® TFE) istypically made as a powder, which is then extruded to form a sheet, andthe sheet is biaxially stretched to form a porous membrane.

A filter membrane may also be secured to a substrate by a method whichinvolves the application of a solvent to which the filter membrane isinert, but which dissolves the substrate. The filter membrane issaturated with the solvent, and then contacted with the substrate. Thecontact of the saturated membrane with the substrate dissolves a portionof the substrate, which is then integrally secured to the membrane afterthe solvent is removed. This method has the severe fault that it may beextremely difficult to maintain a uniform distribution of solventthroughout the filter membrane at the time it is applied to thesubstrate. Simple dipping, or any procedure involving manipulation ofthe wet membrane, invariably leaves more solvent in some portions of themembrane than in others. As a result, an excessively thick bond may formin some areas of contact, while in other areas the bonding between themembrane and the substrate may be inadequate.

For many if not most applications, it is important that the membrane bepositioned precisely at a specific location on the substrate. This isdifficult to do, because the prewetted membrane quite generally is limp,i.e. has no rigidity, and this difficulty is compounded by the rapidevaporation of the solvent, such that a significant loss of solvent canoccur in a few seconds.

Further, in the process described above, the solvent is typicallyallowed to evaporate during the dissolution and bonding process. Thespace within any grooves, which may be present in the substrate, israpidly saturated by the vapor from a small fraction of the solvent and,thus, the bulk of the evaporation takes place at the exposed surface ofthe filter membrane. As solvent evaporates from the exposed surface,solvent from the remainder of the filter membrane migrates bycapillarity through the membrane to the exposed surface. Accordingly,the solvent originally located in contact with the substrate, whichcontains dissolved substrate in solution, also evaporates from theexposed membrane surface. In the process, dissolved substrate may bedeposited at the exposed surface of the filter membrane. This is highlyundesirable, as the pores of the membrane may be at least partiallyclogged by the deposited substrate, locally altering the pore size anddecreasing the permeability of the membrane.

Yet another problem exists with certain supported membranes or porousmedia. For example, the grooves or channels of some supported media tendto be relatively wide. In such instances, the portions of the porousmedium superposed over the channels of the substrate are unsupported.While relatively thick porous media may not be significantly affected inthose regions which are unsupported, when thinner media or membranes areused, large pressure differentials across the porous medium tend to flexand/or distort the medium in the regions of the channels. In thoseinstances in which the pressure differentials across the porous mediumare very high and the tensile strength of the porous membrane is low,the unsupported portions of the medium may not have sufficient pressurepulse resistance to retain its structural integrity and the porousmedium may be breached. Although in some instances a thicker porousmedium with a higher tensile strength may be employed, such an optionmay not exist for many media. Furthermore, where thicker, stronger mediaare available, the pressure drop across the thicker porous medium may betoo high for the intended application. Another alternative is todecrease the width and increase the number of grooves in the surface ofthe substrate in an attempt to obtain the same volume in the grooves.However, there are physical limitations as to the number and depth ofthe grooves which can be used.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a composite filter elementstructure for providing a filtrate or permeate comprises at least oneporous filter medium through which filtrate or permeate passes, asubstrate provided with at least one drainage pathway for passage of thefiltrate or permeate, and at least one support and drainage mediumsandwiched between the porous filter medium and the substrate to conductfiltrate or permeate between the porous filter medium and the substrate.The porous filter medium, the support and drainage medium, and thesubstrate are bonded free of any adhesive, wherein portions of thesubstrate are resolidified within the support and drainage medium andthe porous filter medium without unduly blinding the porous filtermedium.

In another aspect of the present invention, a composite filter elementstructure for providing a filtrate or permeate comprises at least oneporous filter medium through which filtrate or permeate passes, at leastone porous support medium supporting the porous filter medium andallowing filtrate or permeate to pass between the porous filter mediumand the substrate, and a substrate including a drainage mechanism forpassage of the filtrate or permeate. The porous filter medium and theporous support medium communicate with the drainage mechanism of thesubstrate. The porous filter medium and the porous support medium arebonded to the substrate free of any adhesive, wherein portions of thesubstrate are resolidified within the porous support medium and theporous filter medium without unduly blinding the porous filter medium.

The embodiments of the invention represent a considerable advance in thestate of the art. As indicated above, conventional elements are formedby processes which either may not permit a preformed porous medium to besecured to a substrate or, in securing the porous medium to thesubstrate, may substantially alter the porosity or permeability of themedium.

The present invention also provides a uniformly bonded structure and amethod of producing a bonded structure which includes only the filtermembrane, the support and drainage medium and the substrate, therebyavoiding the use of an adhesive component which could leach intofiltrate during use. Further, this invention affords a method ofintegrally securing a preformed porous medium, such as apolytetrafluoroethylene membrane, to a support and drainage medium andin turn to a substrate. In addition, the present invention provides amethod of integrally securing a porous medium to a support and drainagemedium and the latter to a substrate in a manner that does not alter thepore structure or substantially decrease the permeability of the medium.This invention also permits a filter membrane to be bonded to a supportand drainage medium and a substrate with minimal obstruction of edgewiseflow through those portions of the membrane immediately adjacent thebonds. Thus, bonding between adjacent layers is uniform and the porousmedium is precisely located relative to the substrate. In addition,blinding or blockage of the pores of the porous medium is minimized andsignificantly less than conventional elements. In addition, thecomposite structures of the present invention demonstrate issignificantly improved edgewise flow of fluids through the media andresistance to distortion or tearing when exposed to large pressuredifferentials in either direction across the porous medium, either incontinuous or pulsed form. As a result, the composite structures of thepresent invention may be used in high shear applications, such as indynamic filtration and cross-flow filtration.

Another aspect of the present invention provides a method of forming acomposite filter element structure comprising positioning a porousfilter medium to be in communication with a first surface of a supportand drainage medium; positioning a second surface of the support anddrainage medium to be in communication with a surface of a substratehaving at least one fluid pathway to form a component assembly;introducing a bonding composition to the component assembly, the bondingcomposition at least slightly dissolving a portion of the substratesurface without significantly dissolving the porous filter medium or thesupport and drainage medium; contacting the support and drainage mediumand the porous filter medium with the dissolved portion of thesubstrate; and removing the bonding composition and resolidifying aportion of the substrate within the support and drainage medium and theporous filter medium to form an adhesive-free bond between the porousfilter medium, the support and drainage medium, and the substratewithout unduly blinding the porous filter medium.

Another aspect of the present invention provides a method of forming thecomposite structure of the present invention in which a support anddrainage medium is bonded to both a substrate and a porous medium onopposite surfaces of the support and drainage medium. The processincludes contacting a porous medium with a first surface of a supportand drainage medium and contacting the opposite surface of the supportand drainage medium with a surface of a substrate, where the surface ofthe substrate has at least one fluid pathway. Preferably the porousmedium, substrate surface, and support and drainage medium are dry whenthey are placed in contact. A bonding composition is then introduced tothe component assembly formed from the substrate, the support anddrainage medium, and the porous medium. The bonding composition at leastslightly dissolves a portion of the substrate surface without dissolvingthe porous medium or the support and drainage medium and the dissolvedportion of the substrate is contacted with the support and drainagemedium and the porous medium. The bonding composition is then withdrawn,preferably, in a direction opposite to that in which it was introducedto the component assembly to form an adhesive-free bond between theporous medium, the support and drainage medium, and the substrate. Apreferred embodiment includes removing the bonding composition from thecomponent assembly preferably by means of a vacuum.

In the methods of the present invention, the bonding compositionpreferably includes two chemical species having controlled relativevapor pressures. The methods generally include impregnating the porousand support and drainage media with a bonding composition comprising afirst chemical species, which is a solvent for the substrate, and asecond chemical species which is not a solvent for the substrate(non-solvent species). Preferably neither the first chemical species northe second chemical species is a solvent for the porous or supportmedia. The chemical species are preferably selected such that when thebonding composition is removed by a vacuum, the first chemical speciesevaporates faster than the second chemical species.

These above described and other objects and advantages of the presentinvention will be apparent from the description of the invention whichfollows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an oblique view of a section of a substrate of the presentinvention cut perpendicular to grooves in the substrate surface.

FIG. 2 is an oblique view of a section of a composite structure of thepresent invention including the section of the substrate of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a porous, supported composite structureand a method for bonding a porous medium and a support and drainagemedium to a substrate to form such a supported composite structure.Preferably, each of the porous medium and the support and drainagemedium comprise only a single layer, although each medium may comprisetwo or more layers. More particularly, the present invention is directedto a method of integrally bonding a porous filter medium to one surfaceof a support and drainage medium and the opposite surface of the supportand drainage medium to a surface of a substrate and to a supportedcomposite structure produced thereby. Preferably the bond between theporous medium, the support and drainage medium, and the substrate isfree of any adhesive.

The substrate may be any member having sufficient structural integrityto support the porous medium and the support and drainage medium andwhich can be bonded thereto by the method of the present invention. Thesubstrate, which provides support for and defines the configuration ofthe support and drainage medium and the porous medium, may be flexible,semi-flexible or rigid. Further, the substrate includes a material whichat least slightly dissolves in and is solvated by (i.e. absorbs and/oris softened by) the bonding composition. For example, the substratepreferably includes a polymeric material, such as a polyethersulfone, apolysulfone or a polyamide.

The substrate may be a solid structure. If the purpose of the compositestructure is to act as a filter element, the substrate may include amechanism or a pathway for draining fluid away from the substratesurface. The drainage mechanism or pathway may include one, butpreferably a plurality of channels, passages, or interconnecting poresin the substrate surface. In the illustrated embodiment, the drainagemechanism includes at least one groove and preferably a plurality ofgrooves, which may be interconnected, in the substrate surface.Typically, when used in a filter element, the grooves are in fluidcommunication with a filtrate outlet port, which may be coupled to areceiver for filtrate passing through the porous medium. The filteroutlet port may also be used in a preferred method of forming thecomposite structure to introduce bonding composition to the assembledsubstrate and media.

Preferably, the substrate has at least one planar surface in which thegroove(s) are formed. The grooves may be spaced from the edges of thesubstrate, defining flats on the planar surface between the grooves andthe edges. In a preferred embodiment, the substrate is formed as a sheetor plate having opposing planar surfaces. The groove(s) or recessedportion(s) may be formed in one or, preferably, both planar surfaceswith each grooved surface bonded to one surface of a support anddrainage medium and a porous medium bonded to the opposite surface ofthe support and drainage medium.

The porous medium is preferably a porous structure that may be employedas a filter medium and is preferably formed from a polymeric resin butmay include any material capable of forming a porous structure. Thesubstance(s) from which the porous medium, as well as the support anddrainage medium are formed are sufficiently chemically dissimilar to thesubstrate material in a particular composite structure so as to have nosignificant solubility in the bonding composition employed in thepresent invention. Thus, the same substance is most preferably not usedto form a substrate and a support and drainage medium and/or a porousmedium in the same composite supported structure.

The porous medium may comprise any one of a number of materials,including fibrous media made by a variety of means including meltblowing, Fourdrinier deposition, or air laying. The porous medium mayalso comprise porous membrane media made by a variety of means including(i) introducing a solution of a resin in a relatively good solvent intoa solution which is a relatively poor solvent for the resin, e.g., asdescribed in U.S. Pat. No. 4,340,479, (ii) by preparing a solution of aresin in a mixture of two solvents, one of which is a better solventwith a relatively higher vapor pressure compared with the secondsolvent, and allowing the solvents to evaporate, thereby forming aporous film, or (iii) as in the case of Teflon membranes, byprecipitating a suspension of finely particulate PTFE, which is then hotcompressed to form a sheet in which the particles are bonded to eachother, followed by stretching the sheet to form the membrane.

In a preferred embodiment, the porous medium may comprise a microporousfilter medium, such as a microporous fibrous matrix or a microporousmembrane. The method of this invention is particularly useful forsecuring a microporous filter medium to a substrate.

Exemplary porous media may include fluoropolymers, polyamides,polyethersulfones, acrylic polymers, polyesters, or cellulose ester.Preferably, the porous medium includes poly(vinylidene difluoride),polytetrafluoroethylene or a nylon, such as nylon-46, nylon-6, nylon-66or nylon-610. For example, microporous filter media may be preparedusing polyamides following the procedure of U.S. Pat. No. 4,340,479,using poly(vinylidene difluoride) following the procedure of U.S. Pat.Nos. 4,341,615 and 4,774,132, using polytetrafluoroethylene followingthe procedure of U.S. Pat. No. 3,953,566 and 4,096,227, or using apolyethersulfone following the procedure of copending U.S. applicationSer. No. 07/882,473.

When used as filter media, the porous media of the present invention arenot restricted to any particular pore sizes but will depend on theparticular materials being filtered. However, the porous mediapreferably have pore ratings ranging from about 10 nanometers to about10 μm or more, preferably from about 0.04 μm to about 5 μm.

The support and drainage medium or layer is preferably formed from avery open material, allowing fluid to flow laterally and to uniformlydistribute the fluid across the downstream surface of the porous medium.Thus, the support and drainage layer preferably has a very low edgewiseflow resistance. The support and drainage medium to some extentsupplements and provides similar functions to some of those provided bythe substrate. Thus, the support and drainage layer provides the abilityto conduct liquid away from the downstream surface of the porous mediumtoward the drainage pathways of the substrate. In addition, the supportand drainage layer provides additional structural integrity or strengthto the porous medium. Thus, while the substrate provides most of thesupport to the porous medium, in those locations where the porous mediumwould not be supported by the substrate, e.g., because of recesses inthe substrate underlying the porous medium, the support and drainagemedium provides support which increases the resistance to deformation ofthe porous medium into the recessed portions of the substrate. Suchsupport by the support and drainage medium is effective in preventingundue distortion and/or breaches of the porous medium under sustainedhigh pressure conditions or short term pressure pulses in which thepressure drop across the medium involves a significant force on themedia. The support and drainage medium preferably provides support andresists undue distortion and/or breaching of the porous medium atpressure drops in the forward filtration flow direction across themembrane of at least about 25 psid, more preferably of at least about 75psid, and most preferably of more than about 100 psid.

The support and drainage medium should, like the porous medium, besubstantially insoluble in the solvent system used as the bondingcomposition to bond the substrate to the support and drainage medium aswell as to the porous medium. The support and drainage medium shouldalso not be unsuitably affected in any manner by contact with thebonding composition or any liquid medium which it may contact in use.

Any suitable woven or nonwoven material having a relatively coarseporosity and a relatively low edgewise flow resistance compared to theporous medium can be used for the support and drainage medium, withnonwoven materials being generally preferred. Typically, such supportand drainage media have air permeabilities from about 70 scfm to about1500 scfm at 1/2 inch water. Preferably, the support and drainage mediumused in the present invention should have a tensile strength of at leastabout 75 lbs/in² (5.2 kgs/cm²) and a thickness of about 1 to about 20mils (about 25 microns to about 0.51 mm), more preferably from about 3to about 10 mils. The air permeability of the support and drainagemedium is preferably at least about 70 scfm/ft² of airflow at one-halfinch of water, most preferably about 100 to 300 scfm/ft². Natural fibersor polymeric materials may be employed to form the support and drainagemedium, with certain polymeric materials being preferred. Preferably thesupport and drainage medium is formed from a polyolefin, such aspolypropylene, or a polyester, such as polyethylene terephthalate.However, polyamides, aramids and glass fibers may also be used.

Preferred nonwoven support and drainage media include a polyethyleneterephthalate polyester web available from Hirose® as 05TH15, apolypropylene web available from Hirose® as HOP30H, and spunbondedpolypropylene media available from Midwest Filtration Company as Unipro.

Woven support and drainage media are less preferred than nonwoven mediabecause they typically are thicker and have a less uniform surface andhigh edgewise flow resistance. However, woven support and drainage mediatypically have greater strength than nonwoven media so they may bepreferable for substrates with wide grooves or in high pressureenvironments. A preferred woven material is a polyester mesh availablefrom Tetko® as mesh no. 7-105/52.

As shown in FIG. 1, the portion of the substrate 10 between adjacentgrooves 12 is referred to hereinafter as the crest 11. The grooves 12may have any suitable configuration, such as a semicircularconfiguration, a V-shaped configuration or the generally U-shapedconfiguration shown in FIG. 1. Similarly, the crests 11 may have avariety of configurations, such as the apex of the angle betweenclosely-spaced V-shaped grooves or, more preferably, a flat planarsurface between more widely spaced grooves. The crests 11 constitutemuch of the substrate surface to which the support and drainage medium13 and the porous medium 5 are secured.

As shown in FIG. 2, the porous medium 15, the support and drainagemedium 13, and the substrate are bonded together, preferably bydissolved substrate free of any adhesive. The support and drainagemedium 13 is integrally secured to the substrate surface 14 by forming abond between the crests 11 and the support and drainage medium 13. Inthe composite structure of the present invention, the porous medium 15is also integrally bonded to the support and drainage medium 13 by abond formed between the surface portions 16 of the two media. Typically,the dissolved substrate material which bonds the substrate 10 to thesupport and drainage medium 13 and the support and drainage medium 13 tothe porous medium 15 extends across the thickness of the support anddrainage medium 13 from one bonded surface to the other bonded surface.Thus, to provide an effective bond, the thickness of the support anddrainage medium 13 is preferably less than about 20 mils (0.51 mm). Thedepth of penetration of the bond 16 into the porous medium 15 maypreferably be a very small fraction of the thickness of the porousmedium 15, as this permits the portion of the porous medium 15 above thecrests 11 to function effectively without blinding by allowing edgewiseflow. This is however, much less of a problem with the compositestructure of the present invention as compared with conventionalelements which lack the support and drainage medium of this invention.

An embodiment of the present invention also comprises a method forbonding a porous medium and a support and drainage medium to a substratewhich includes, for example, contacting a porous medium with one surfaceof a support and drainage medium and the opposite surface of the supportand drainage medium with a surface of a substrate so that the supportand drainage medium is sandwiched between the porous medium and thesubstrate. Preferably this is done when both media and the substratesurface are dry. The support and drainage medium and at least a portionof the porous medium are then impregnated with a bonding compositionwhich at least slightly dissolves the substrate surface withoutdissolving the porous medium or the support and drainage medium.

Initially, the components of the composite structure (e.g., thesubstrate, the support and drainage medium, and the porous medium) arelocated with the support and drainage medium sandwiched between theporous medium and the substrate. In composite structures in which thesubstrate includes fluid pathways or grooves on opposing surfaces, thesubstrate is sandwiched between two support and drainage media, and thefirst support and drainage medium, the substrate, and the second supportand drainage medium are, in turn, sandwiched between two porous media.Each surface of the substrate then contacts one surface of a support anddrainage medium while the opposite surface of the support and drainagemedium contacts one surface of a porous medium.

The components of the composite structure are placed between clampingplates of a rigid, liquid impervious material, such as aluminum. In manyinstances, it is desirable to place a porous pad between the clampingplate and the porous medium in contact with the outer surface of theporous medium. The material from which the porous pad is formed hasproperties, including the non-solubility in bonding composition, similarto those of the support and drainage medium used in the presentinvention. The thickness of the pad, however, is preferably much greaterthan that of the support and drainage medium used in a particularapplication so the porous pad may contain a greater volume of thebonding composition. For example, the porous pad may contain from 1 to30 times the volume of the bonding composition compared to the porousmedium and the support and drainage medium. Materials suitable for useas a porous pad include the same materials used for support and drainagemedia. In addition, a coarse mesh may be positioned between the clampingplate and the porous pad. The apparatus, as well as many of thetechniques, used to form the composite structure of the presentinvention are analogous to those described in commonly owned, co-pendingInternational Application No. PCT/US94/03104 filed 23 Mar. 1994.

Once the coarse mesh, porous pad(s) and the components of the compositestructure are placed between the clamping plates, the clamping platesare activated to compress the entire assembly. In a preferredembodiment, pressure is applied to force the coarse mesh, the porouspad, the porous medium, the support and drainage medium, and thesubstrate together, compressing the assembly and ensuring that theporous medium, the support and drainage medium, and the substrate are infirm contact. The pressure applied by the clamping plates varies withthe nature of the substrate and the media used. The pressure suitablyranges from bare contact to about 100 psi. Preferably the pressure isabout 2 to about 25 psi and most preferably is about 5 to about 7 psi.

Once the assembly is clamped together, a bonding composition isintroduced to the clamped assembly to bond the porous medium, thesupport and drainage medium, and the substrate. The bonding compositionpreferably is free of any adhesive and comprises a mixture of at leasttwo chemical species, the first chemical species being a good solventfor a substrate and the second chemical species being a non-solvent forthe substrate. Preferably, neither the solvent species nor thenon-solvent species is a solvent for the support and drainage medium orthe porous medium. The starting constitution of the bonding compositionmay vary from 100% solvent species and 0% non-solvent species to about10% solvent species and 90% non-solvent species by weight. Morepreferably, the starting composition is in the range from about 70%solvent species and 30% non-solvent species to about 30% solvent speciesand 70% non-solvent species.

Exemplary chemical species which may be used as a solvent speciesinclude but are not limited to halogenated hydrocarbons, such asmethylene chloride or chloroform. Preferably, the solvent speciesincludes methylene chloride. Exemplary chemical species which may beused as the non-solvent species include but are not limited to alcoholsand hydrocarbons. Preferably, the non-solvent species is methanol,cyclopentane, polymethyl pentane. Exemplary bonding compositions forbonding a polyamide, a poly (vinylidene fluoride) or apolytetrafluoroethylene porous medium and a polyester or polyethylenesupport and drainage medium to a polyethersulfone or polysulfonesubstrate include mixtures of methylene chloride as the solvent speciesand methyl alcohol, polymethyl pentane, or cyclopentane as thenon-solvent species.

The bonding composition may be introduced to the clamped assembly in anysuitable manner. In a preferred method, the bonding composition may beintroduced first to the substrate whence it flows to and contacts thesupport and drainage medium, ultimately impregnating and saturating thesupport and drainage medium. The bonding composition then contacts atleast the surface of the porous medium contacting the support anddrainage medium. Preferably the bonding composition also saturates theporous medium, the porous pad, and the coarse mesh.

As indicated above, the substrate is provided with a mechanism or apathway for draining filtrate away from the substrate surface, such aschannels or grooves. These channels or grooves may also serve as thepath of the bonding composition along the substrate to the support anddrainage medium and the porous medium and hence to the porous pad andthe coarse mesh. Frequently, particularly when the substrate is a rigidsolid material of significant thickness, the substrate is also providedwith a filtrate outlet port or permeate port which is in fluidcommunication with the grooves in the substrate surface. When such aport is provided in the substrate, the bonding composition may beintroduced to the substrate, for example, using a syringe or a syringepump and/or an appropriate fitting in the port. While the orientation ofthe clamped assembly does not appear to be critical, during theintroduction of the bonding composition when the bonding composition isintroduced from the direction of the substrate and flows to the porousmedium, it is generally preferred to orient the clamped assembly in avertical position with the permeate port at the top.

The impregnated porous medium, the impregnated support and drainagemedium, and the substrate surface are maintained in contact until thesubstrate surface is at least slightly solvated by or slightly dissolvedin the bonding composition and the dissolved substrate contacts thesupport and drainage medium and the porous medium. The bondingcomposition and the dissolved substrate wicks along the support anddrainage medium between the porous medium and the crests and flats ofthe substrate and into the porous medium. Preferably, thecharacteristics of the bonding composition are selected or adjusted toobtain a satisfactory degree of bonding during a hold period of at leastabout 15 to 25 seconds. ("Hold period" refers to the time during whichthe porous medium, the impregnated support and drainage medium, and thesubstrate are maintained in contact from the introduction to the removalof the bonding composition.) Still longer hold periods, such as fromabout 100 to about 150 seconds or more, are more preferred.

The optimum duration of the hold period is preferably determinedempirically for a specific bonding composition. Generally, the bondstrength increases but the permeability of the porous medium decreaseswith increasing hold time. For example, the final composite structuremay be tested by passing water therethrough in the normal filtrationflow direction (i.e., from the porous medium through the support anddrainage medium to the substrate) in order to determine what percentageof the permeability of the porous medium has been lost. This percentagebecomes higher as the hold period is increased and more of the dissolvedsubstrate wicks into the porous medium. The composite structure may alsobe tested by flowing water in the reverse filtration flow direction, inorder to determine the pressure at which the porous medium is duelydistorted or separates from the support and drainage medium and/or thelatter from the substrate. Several specimens can be made using a givenbonding composition and various hold periods. The test data derived fromthese specimens may then be used to select an optimum hold period.

The optimum hold period varies greatly depending on the particularchemical species used to prepare the bonding composition. Because thebonding composition may be compounded using a combination of anaggressive solvent species with a non-solvent species, the degree ofsolvency of the substrate in the bonding composition, and hence the holdperiod required, may be adjusted by varying the proportions of the twospecies.

When the bonding composition is introduced to the clamped assembly, someparts of the porous medium, the support and drainage medium and thesubstrate are unavoidably wetted by the bonding composition before otherparts. For example, if the substrate, support and drainage medium, andporous medium being bonded are quite large, some parts may be exposedfor as much as 15 seconds or more longer than other parts. If thebonding composition is selected or compounded such that the hold periodis about 15 seconds, then some parts of the porous medium in contactwith the support and drainage medium may have been exposed for twice aslong as others. This may lead to overbonding of one section of theresulting composite structure with the flow of filtrate through thecomposite structure inhibited locally, while another section may fail inthe reverse filtration flow mode.

A bonding composition comprising a mixture of chemical species makespossible relatively longer hold periods. As noted above, the advantageof longer hold periods is that the effect of the differential wettingwhich can occur during the filling operation is minimized. Whencompared, for example, with the same 15 second wetting differential ofthe preceding paragraph, the use of a mixture of chemical speciescomposition for which bonding is optimized by a 150 second hold periodreduces the difference between the longest and shortest total timeduring which any part of the substrate is in contact with theimpregnated porous medium prior to flushing to about 10% of the holdperiod.

After the hold period, the bonding composition is then withdrawn orremoved from the clamped assembly. For example, the bonding compositionmay be withdrawn in a direction opposite to that in which it wasintroduced, i.e., from the porous medium through the support anddrainage medium and then through the substrate, e.g. along the filtratepathway and out the filtrate port. As soon as the desired hold periodbetween the porous medium, the support and drainage medium and thesubstrate has been reached, the bonding composition contained in thecoarse mesh and in the porous pad may be rapidly flushed through theporous medium and the support and drainage medium, preferably in thedirection of the substrate and out of the clamped apparatus via thegrooves and the filtrate port of the substrate. For example, the bondingsolution may be removed by applying air or other gas pressure at thecoarse mesh or the porous pad or by applying a vacuum to the side of theporous medium closest to the substrate, e.g., by applying a vacuum tothe substrate outlet port. The application of pressure or vacuum maythen be continued until the now bonded structure is dry (i.e., theresidual bonding composition has been evaporated), in the one case byevaporation into the introduced gas and in the other by evaporation ofthe solvent into the vacuum. As the solvent species is evaporated fromthe bonding composition, the dissolved substrate precipitates andsolidifies within the support and drainage medium and the porous mediumand on the surface of the substrate, mechanically entangling andgenerating a strong, secure bond between the substrate, the support anddrainage medium, and the porous medium.

The rapid flushing of bonding composition from the coarse mesh and theporous pad through the porous medium towards the substrate isbeneficial, as it removes some of the dissolved substrate from theporous medium. Allowing all of the dissolved substrate to remain inplace could unduely blind the porous medium and partially obstructfiltrate flow in the porous medium.

It is desirable to reduce as much as possible differences in exposuretime in this stage between one part of the bonded surface and another.This may be accomplished in part by applying a high degree of vacuum atthe conclusion of the hold period, thereby rapidly removing the bondingcomposition by evaporation as the bonding composition is being flushedthrough the porous medium. The effectiveness of this procedure is,however, hampered by the absorption of heat during vaporization whichcools the chemical species contained in the bonding composition,reducing their vapor pressure and the effective pumping rate. However,the non-solvent species may be selected to have a lower vapor pressurethan the solvent species of the bonding composition, preferably by about10% or more at ambient temperature. When the vacuum is applied, thesolvent species is removed faster than the non-solvent species, therebydecreasing the concentration of the solvent species in the residualbonding composition. Preferably, the starting constitution of thebonding composition is chosen such that the residual bonding compositionbecomes a non-solvent for the substrate after a very short period ofevaporation, thereby preventing any further dissolution of the substrateand limiting the time during which dissolution of the substrate occursto a very short period after exposure to the vacuum, which may be asshort as about 5 seconds or less.

Various alternative methods also embody the present invention. As onealternative method, bonding composition may be introduced from thedirection of the coarse mesh through the porous pad to the porous mediumand then through the support and drainage medium to the surface of thesubstrate. As with the method described immediately above, the bondingcomposition is allowed to remain in the porous and the support anddrainage media, both of which are impregnated, and in contact with thesurface of the substrate for a suitable hold period. As anotherembodiment, the bonding composition may be removed from the compressedassembly in the direction of the coarse mesh or porous pad using apressure differential, for example, by applying a vacuum at the coarsemesh or the porous pad.

Test Methods

The following non destructive test methods are conducted on compositestructures comprising microporous membrane filter elements made bymethods of this invention. The filter elements are generally tested in aleak-tight assembly having seals which separate the upstream side fromthe downstream side of the filter element.

Bond Strength (Reverse pressure): The bond strength between individualcomponents and the adjacent component to which they are bonded in thecomposite structure, i.e., the porous medium, the support and drainagemedium, and the substrate, can be determined by applying pressure in thereverse filtrate flow direction. Pressure is increased incrementallyuntil the bond between the porous medium, the support and drainagemedium, and the substrate fails. If no evidence of bond failure isobserved at 5 psi for a 60 second dwell time, the relative bond strengthis deemed acceptable.

Permeability (Flow ΔP): The effective permeability of a porous mediumcan be determined by measuring the flow of water as a function ofapplied pressure. Using water at ambient temperature, which has beenpreviously passed through a 0.04 μm filter, the filtrate flow andpermeability is measured in the forward filtrate flow direction at 2.5,5.0, and 10.0 psi. The data is reported as an average flow rate in unitsof mil/min/psi.

Porosity Bubble Point Test: Each membrane filter element is tested forporosity by using a bubble point test as described in ASTM F316-86.

Forward Pressure/Temperature Rating: The relative forwardpressure/temperature rating of a porous medium bonded to a substrate canbe obtained by applying pressure in the filtrate forward flow direction,at a given temperature, until the porous medium begins to yield or grossfailure is observed. Hot water filtered through a 0.1 μm filter ispumped through the composite structure at 60 psi pressure and 90° C. for30 minutes.

EXAMPLES Example 1

A 1/4-inch thick, 16 inch diameter semicircular injection moldedpolysulfone disk served as the substrate. The semicircular disk wasprovided with a series of concentric grooves on both sides that drain toa single central channel and permeate port. A wet laid polyesternonwoven fibrous web available under the trade designation HIROSE 05TH15was used as the support and drainage medium. An ULTIPOR® N₆₆ polyamide0.45 μm rated microporous membrane available from Pall Corporation, EastHills, N.Y. served as the porous medium. The support and drainage mediumand porous medium were cut to dimensions such that once the support anddrainage medium and the porous medium were each positioned on andadjacent the substrate, respectively, the entire grooved area of thesubstrate was covered, extending past the peripheral grooves onto theflats by 0.300 inch (7.6 mm). The substrate was sandwiched between alayer of support and drainage medium and a layer porous medium on bothsides, with the support and drainage medium layers positioned closest tothe substrate. This layered assembly was then placed between a pair ofporous pads consisting of 10 layers of spunbond polypropylene nonwovenavailable under the trade designation LUTRASIL LSVP 688 and a pair of3/4" aluminum plates and clamped at 7 psi clamping pressure. Thepermeate port, which extended out from the clamping plates, was equippedwith an O-ring sealed stainless steel fixture to permit the bondingcomposition to be injected and evacuated from the permeate port. Thebonding composition consisted of a solvent/non-solvent mixture of 54%methylene chloride and 46% cyclopentane, by weight. With the clampedassembly in the vertical position, 150 ml of bonding composition wasinjected quickly through the permeate port with a glass syringe, fillingthe grooves of the substrate, the support and drainage media, the porousmedia and the porous pads with bonding composition and expelling theair. Once the bonding composition was injected, a hold time of 120seconds was initiated. After 120 seconds elapsed, the excess bondingcomposition was evacuated by applying vacuum for 15 minutes at thepermeate port. After 15 minutes, evacuation was discontinued and thecomposite structure comprising a filter element was removed from theclamped plates. The bond strength was tested by applying a reversepressure of 5 psi for 60 seconds at the permeate port with no evidenceof membrane failure, indicating that the porous medium was integrallybonded to the substrate. The flow ΔP and bubble point were determined tobe 1215 ml/min/psi and 31.5 psi, respectively. The flow ΔP and thebubble point of the porous medium prior to bonding were determined to be1550 ml/min/psi and 31.0 psi, respectively. Thus, the compositestructure retained 78% of the porous medium effective permeability withno substantial alteration in pore size as a result of the bonding methoddescribed.

Example 2

A 1/4-inch thick, 6" diameter, circular injection molded polysulfonedisk of the type used in Example 1 but having a series of concentricgrooves on only one side and draining to a single central channel andpermeate port served as the substrate. As in Example 1, a wet laidpolyester nonwoven fibrous web available under the trade designationHIROSE 05TH15 served as the support and drainage medium, and an ULTIPORN₆₆ ® polyamide 0.45μ rated microporous membrane available from PallCorporation served as the porous medium. The support and drainage mediumand porous medium were cut to dimensions such that once the support anddrainage medium and porous medium were positioned on the substrate, theentire grooved area of the substrate was covered, extending past theperipheral grooves onto the flats by 0.300 inch. The support anddrainage medium was sandwiched between the substrate and the porousmedium on the grooved side, with the support and drainage mediumpositioned closest to the substrate. This layered assembly was thenpositioned between a pair of 1/8" aluminum plates, a porous padconsisting of 10 layers of a spunbond polypropylene nonwoven availableunder the trade designation LUTRASIL LSVP 688 was placed between theporous medium and one of the clamping plates and the assembly wasclamped together at 8 psi clamping pressure. The permeate port, locatedon the ungrooved side of the substrate, was equipped with aninterference fit polypropylene luer fitting to permit the bondingcomposition to be injected and evacuated from the permeate port. Thebonding composition consisted of a solvent/non-solvent mixture of 54%methylene chloride and 46% cyclopentane, by weight. With the clampedassembly in the horizontal position and the grooves in the upper surfaceof the substrate, 45 ml of bonding composition were injected quicklythrough the permeate port with a glass syringe, filling the grooves ofthe substrate, the support and drainage medium, the porous medium, andthe pad with bonding composition and expelling the air. Once the bondingcomposition was injected, a hold time of 150 seconds was initiated.After 150 seconds elapsed, the excess bonding was evacuated by applyinga vacuum for 2 minutes at the permeate port. After 2 minutes, evacuationwas discontinued and the composite structure comprising a filter elementwas removed from between the clamped plates. Flow ΔP and bubble point ofthe composite structure were measured at 260 ml/min/psi and 31 psi,respectively. Flow ΔP and bubble point of the unbonded porous mediumwere measured at 313 ml/min/psi and 31 psi, respectively. Thus, thecomposite structure retained 83% of the porous medium effectivepermeability with no alteration in pore size. The bond strength wastested by applying a reverse pressure of 5 psi for 60 seconds at thepermeate port with no evidence of membrane failure, indicating that theporous medium was integrally bonded to the substrate.

Example 3

A polytetrafluoroethylene 0.2μ rated microporous membrane, serving asthe porous medium, was bonded with a polyester nonwoven support anddrainage medium and a polysulfone disk of the type and by the generalmethod described in Example 2. The bond strength was tested by applyinga reverse pressure of 5 psi for 60 at the permeate port with no evidenceof membrane failure, indicating that the porous medium was integrallybonded to the substrate. The bubble point pressure of the porous mediummeasured in alcohol before and after bonding was 15.6 and 15.5 psi,respectively, indicating no alteration in pore size. The flow ΔP of thecomposite structure was measured at 194 ml/min/psi.

Example 4

An ULTIPOR® N₆₆ polyamide 0.45 μm rated microporous membrane availablefrom Pall Corporation served as the porous medium, and a woven polyestermesh available under the trade designation TETKO PeCap 7-105/52 servedas the support and drainage medium. The porous medium and the supportand drainage medium were bonded to a polysulfone disk of the type andgenerally by the method described in Example 2. A volume of 32 ml ofbonding composition was injected into the permeate port and a hold timeof 200 seconds was used. After 200 seconds elapsed, the excess bondingcomposition was evacuated by applying a vacuum for 3 minutes at thepermeate port. After 3 minutes, evacuation was discontinued and thecomposite structure was removed from between the clamped plates. Thebond strength was tested at a reverse pressure of 5 psi for 60 secondsat the permeate port with no evidence of membrane failure, indicatingthat the porous medium was integrally bonded to the substrate. Flow ΔPand bubble point of the composite structure element were measured at 246ml/min/psi and 31 psi respectively. Flow ΔP and bubble point of theunbonded porous medium were measured at 314 ml/min/psi and 31 psi,respectively. Thus, the composite structure retained 79% of the porousmedium effective permeability with no alteration in pore size.

Example 5

A composite structure was prepared as described in Example 2 and testedto establish a forward pressure/temperature rating. The compositestructure was exposed to 90° C. filtered deionized water at 60 psiforward pressure for 30 minutes. The bond strength was also tested at areverse pressure of psi for 60 seconds applied at the permeate port withno evidence of membrane failure, indicating that the porous mediumremained integrally bonded to the substrate. The flow ΔP and bubblepoint of the composite structure before the exposure were measured at250 ml/min/psi and 30.0 psi, respectively. After the exposure, the flowΔP and bubble point were measured at 270 ml/min/psi and 30.5 psirespectively, indicating no significant alteration in permeability orpore size. No evidence of membrane yield was observed.

As shown in the previous disclosure and examples, a composite structureembodying the present invention has many advantages, in particular, thecomposite structure may comprise a highly superior filter element. Forexample, filter element is capable of withstanding reverse pressures of5 psi and forward pressure drops across the porous mediium of at leastabout 25 psid. Further, the bond between the porous medium, the supportand drainage medium, and the substrate is exceedingly strong, enablingthe porous medium to withstand sheer rates of about 5000 per second. Notonly is the bond exceedingly strong bu the permeability of the bondedporous media remains exceedingly high. For example, permeability of thebonded porous medium is at least about 50% of the permeability of theunbonded porous medium. In addition, the porosity of the bonded porousmedium is substantially unchanged from the porosity of the onbondedporous medium.

Another advantage of filter elements embodying the present invention isthat the bond between the porous medium, the support and drainagemedium, and the substrate is formed without any adhesive by thesolidification of the dissolved substrate. Consequently, there is noadhesive to leech into the filtrate and the bond is not effected byaggressive chemicals unless the chemicals are capable of attacking thesubstrate. Further, where the substrate is formed from a hightemperature polymeric material, the bond remains intact at elevatedtemperatures until the softening point of the substrate is reached. Fora high temperature polymer such as polysulfone, operating andsterilization temperatures may be as great as 250 degrees celcius.

Although the present invention has been described in terms of exemplaryembodiments, it is not limited to these embodiments. Alternativeembodiments, examples, and modifications which would still beencompassed by the invention may be made by those skilled in the art,particularly in light of the foregoing teachings. Therefore, thefollowing claims are intended to cover any alternative embodiments,examples, modifications, or equivalents which may be included within thespirit and scope of the invention as defined by the claims.

We claim:
 1. A composite filter element structure for providing afiltrate or permeate, the composite filter element structurecomprising:a porous filter medium through which filtrate/permeatepasses; a substrate provided with at least one drainage pathway forpassage of the filtrate/permeate; and a support and drainage mediumsandwiched between the porous filter medium and the substrate to conductfiltrate/permeate between the porous filter medium and the substrate;the porous filter medium, the support and drainage medium, and thesubstrate being bonded free of any adhesive, wherein portions of thesubstrate are resolidified within the support and drainage medium andthe porous filter medium without unduly blinding the porous filtermedium.
 2. The composite filter element structure according to claim 1wherein the porous filter medium includes a polyamide, fluoropolymer,polyethersulfone, acrylic polymer, polyester or cellulose ester.
 3. Thecomposite filter element structure according to claim 1 wherein thesubstrate comprises polysulfone, polyethersulfone, or polyamide.
 4. Thecomposite filter element structure according to claim 1 wherein thesupport and drainage medium includes a woven material.
 5. A method offorming a composite filter element structure comprising:positioning aporous filter medium to be in communication with a first surface of asupport and drainage medium; positioning a second surface of saidsupport and drainage medium to be in communication with a surface of asubstrate having at least one fluid pathway to form a componentassembly; introducing a bonding composition to said component assembly,said bonding composition at least slightly dissolving a portion of thesubstrate surface without significantly dissolving the porous filtermedium or the support and drainage medium; contacting the support anddrainage medium and the porous filter medium with the dissolved portionof the substrate; and removing the bonding composition andresolidifying, a portion of the substrate within the support anddrainage medium and the porous filter medium to form an adhesive-freebond between the porous filter medium, the support and drainage medium,and the substrate without unduly blinding the porous filter medium. 6.The method of claim 5 further comprising applying pressure to compressthe porous filter medium, the support and drainage medium, and thesubstrate.
 7. The method of claim 5 wherein positioning the support anddrainage medium to be in communication with the substrate surfacecomprises positioning the support and drainage medium to support theporous filter medium over the fluid pathway of the substrate.
 8. Themethod of claim 5 wherein positioning the porous filter medium to be incommunication with the first surface of the support and drainage mediumcomprises contacting the porous filter medium with the first surface ofthe support and drainage medium.
 9. The method of claim 5 whereinpositioning the second surface of the support and drainage medium to bein communication with the surface of the substrate comprises contactingthe second surface of the support and drainage medium with thesubstrate.
 10. The method of claim 5 wherein introducing the bondingcomposition comprises introducing a bonding composition which includes afirst chemical species comprising a solvent for the substrate and asecond chemical species comprising a non-solvent for the substrate. 11.The method of claim 5 wherein introducing the bonding compositioncomprises introducing the bonding composition to the porous filtermedium before the support and drainage medium.
 12. The method of claim 5wherein introducing the bonding composition comprises introducing thebonding composition to the substrate before the support and drainagemedium.
 13. The method of claim 12 wherein introducing the bondingcomposition to the substrate before the support and drainage mediumcomprises introducing the bonding composition into a permeate outletport of the substrate.
 14. The method of claim 12 wherein introducingthe bonding composition to the substrate before the support and drainagemedium comprises introducing the bonding composition along one or moregrooves in the substrate surface.
 15. The method of claim 5 wherein theporous filter medium and the support and drainage medium are slightlysoluble in the bonding composition.
 16. The method of claim 5 whereincontacting the support and drainage medium and the porous filter mediumwith the dissolved portion of the substrate comprises contacting thedissolved portion of the substrate with a non-woven fibrous support anddrainage medium and the porous filter medium.
 17. The method of claim 5wherein contacting the support and drainage medium and the porous filtermedium with the dissolved portion of the substrate comprises contactingthe support and drainage medium and the porous filter medium with thedissolved portion of the substrate while the component assembly isenclosed within a sealed chamber.
 18. The method of claim 5 whereinremoving the bonding composition comprises applying a vacuum.
 19. Themethod of claim 5 wherein removing the bonding composition comprisesremoving the bonding composition in the direction of the porous filtermedium.
 20. The method of claim 19 wherein removing the bondingcomposition in the direction of the porous filter medium comprisesapplying a vacuum in the direction of the porous filter medium.
 21. Themethod of claim 5 wherein removing the bonding composition comprisesremoving the bonding composition in the direction of the substrate. 22.The method of claim 21 wherein removing the bonding composition in thedirection of the substrate comprises removing the bonding compositionthrough a permeate outlet port of the substrate.
 23. The method of claim22 wherein removing the bonding composition through the permeate outletport of the substrate comprises directing the bonding composition thoughone or more grooves in communication with the permeate outlet port. 24.The method of claim 23 wherein directing the bonding composition thoughone or more grooves in communication with the permeate outlet portcomprises applying a vacuum to the permeate outlet port.
 25. The methodof claim 8 wherein positioning the second surface of the support anddrainage medium to be in communication with the surface of the substratecomprises contacting the second surface of the support and drainagemedium with the substrate.
 26. The method of claim 25 further comprisingapplying pressure to compress the porous filter medium, the support anddrainage medium, and the substrate.
 27. The method of claim 26 whereinremoving the bonding composition comprises applying a vacuum.
 28. Themethod of claim 27 wherein contacting the support and drainage mediumand the porous filter medium with the dissolved portion of the substratecomprises contacting the dissolved portion of the substrate with anon-woven fibrous support and drainage medium and the porous filtermedium.
 29. The method of claim 28 wherein introducing the bondingcomposition comprises introducing a bonding composition which includes afirst chemical species comprising a solvent for the substrate and asecond chemical species comprising a non-solvent for the substrate. 30.The method of claim 27 wherein contacting the support and drainagemedium and the porous filter medium with the dissolved portion of thesubstrate comprises contacting the support and drainage medium and theporous filter medium with the dissolved portion of the substrate whilethe component assembly is enclosed within a sealed chamber.
 31. Themethod of claim 30 wherein introducing the bonding composition comprisesintroducing a bonding composition which includes a first chemicalspecies comprising a solvent for the substrate and a second chemicalspecies comprising a non-solvent for the substrate.
 32. The compositefilter element structure of claim 1 wherein the drainage pathway of thesubstrate has a plurality of grooves, and the support and drainagemedium is bonded to the portion of the substrate between adjacentgrooves.
 33. The composite filter element structure of claim 1 whereinthe porous filter medium comprises two or more layers.
 34. The compositefilter element structure of claim 1, wherein the substrate is formed asa sheet or plate having opposing surfaces and each surface is bondedfree of any adhesive to a support and drainage medium and a porousfilter medium.
 35. A composite filter element structure for providing afiltrate of permeate, the composite filter element structurecomprising:a porous filter medium through which filtrate/permeatepasses; a porous support medium supporting the porous filter medium andallowing filtrate/permeate to pass between the porous filter medium andthe substrate; and a substrate including a drainage mechanism forpassage of the filtrate/permeate; wherein the porous filter medium andthe porous support medium communicate with the drainage mechanism of thesubstrate and wherein the porous filter medium and the porous supportmedium are bonded to the substrate free of any adhesive, whereinportions of the substrate are resolidified within the porous supportmedium and the porous filter medium without unduly blinding the porousfilter medium.
 36. The composite filter element structure of claim 35wherein the porous filter medium includes a microporous filter medium.37. The composite filter element structure of claim 35 wherein theporous filter medium includes a polymeric material.
 38. The compositefilter element structure of claim 37 wherein the porous filter mediumincludes a polyamide, fluoropolymer, or polyethersulfone.
 39. Thecomposite filter element structure of claim 35 wherein the porous filtermedium comprises two or more layers.
 40. The composite filter elementstructure of claim 35 wherein the substrate comprises a polymericmaterial.
 41. The composite filter element structure of claim 40 whereinthe substrate comprises polyethersulfone, polysulfone, or polyamide. 42.The composite filter element structure of claim 35 wherein the drainagemechanism of the substrate has a plurality of grooves, and the poroussupport medium is bonded to the portion of the substrate betweenadjacent grooves.
 43. The composite filter element structure of claim 42wherein the porous support medium bridges the adjacent grooves of thesubstrate.
 44. The composite filter element structure of claim 35wherein the porous filter medium and the porous support medium arebonded to the substrate by solidified substrate contained within theporous support medium and the porous filter medium.
 45. The compositefilter element structure of claim 35, wherein the substrate is formed asa sheet or plate having opposing planar surfaces and each planar surfaceis bonded free of any adhesive to a porous support medium and a porousfilter medium.
 46. The composite filter element structure of claim 35wherein the substrate is rigid.
 47. The composite filter elementstructure of claim 35 wherein the porous support medium includes apolymeric material.
 48. The composite filter element structure of claim47 wherein the porous support medium includes a non-woven fibrousmaterial.
 49. The composite filter element structure of claim 48 whereinthe porous support medium has a thickness of less than 20 mils.
 50. Thecomposite filter element structure of claim 49 wherein the poroussupport medium has low edgewise flow resistance.
 51. The compositefilter element structure of claim 35 wherein the porous support mediumincludes a non-woven fibrous material.
 52. The composite filter elementstructure of claim 35 wherein the porous support medium has a thicknessof less than 20 mils.
 53. The composite filter element structure ofclaim 35 wherein the porous support medium has low edgewise flowresistance.